1. Field of the Invention
The present invention relates to a field emission cold cathode element, and particularly to a field emission cold cathode element having a current control element connected to an emitter.
2. Description of the Related Art
A field emission cold cathode element is an element including sharp, cone-shaped emitters and submicron openings, and that focuses a high electric field at the emitter tips by means of a gate electrode formed in proximity to the emitters, thereby emitting electrons from the emitter tips into a vacuum. This type of field emission cold cathode element has the problem that discharges may occur during operation between the emitters and the gate or anode electrode due to, for example, the effect of gas, and as a result of such discharges, a large current flows to emitters, the emitter material fuses, and short circuits occur between the emitters and gate.
In addition, the adhesion of minute extraneous particles to the element may cause short circuits between the gate and emitters and a rise in emitter potential. As a countermeasure, elements have been developed in which resistors are added in series to the emitters to control the current of discharges and thereby prevent fusing of the emitters. Such methods, however, entail the drawback of increases in operation voltage due to drops in potential in the resistance layer even during normal operation when discharges do not occur.
A method of forming an active element at the emitters having a saturation current characteristic has also been proposed as a method of controlling current flowing to emitters.
This type of field emission cold cathode element is explained hereinbelow with reference to the accompanying figures.
As shown in FIG. 1, the first example of the prior art is made up of sharp, cone-shaped emitter 106 composed of, for example, molybdenum; gate electrode 107 composed of tungsten formed so as to surround emitter 106, insulation film 108 composed of an oxide film formed below gate electrode 107, n-type silicon 103 connected to emitter 106; p-type silicon 105 formed so as to surround n-type silicon 103; p-type lead electrode 113 composed of tungsten and connected to p-type silicon 105; n-type silicon substrate 101 connected to n-type silicon 103 and p-type silicon 105; and substrate electrode 109 connected to n-type silicon substrate 101.
In a field emission cold cathode element constructed according to the foregoing description, a junction-type field effect transistor is formed from n-type silicon 103, p-type silicon 105, and n-type silicon substrate 101; and current flowing within n-type silicon 103 can be controlled by varying voltage impressed to p-type silicon 105. In addition, to ensure dielectric strength, the concentration of n-type impurity in n-type silicon 103 is set to substantially the same level as the concentration of p-type impurity in p-type silicon 105, and the depth of n-type silicon 103 is set to exceed a value obtained by dividing twice the voltage impressed between emitter 106 and n-type silicon substrate 101 by the breakdown field intensity.
In the second example of the prior art, as shown in FIG. 2, a bipolar transistor is formed from n-type silicon 203 formed below emitter 206 and p-type silicon 214 formed below n-type silicon 203. The current flowing from n-type silicon 203, which constitutes the emitter of the bipolar transistor, to n-type silicon substrate 201, which constitutes the collector of the bipolar transistor, can be controlled by varying the voltage impressed to p-type silicon 214, which constitutes the base of the bipolar transistor.
Nevertheless, above-described field emission cold cathode elements of the prior art have the following drawbacks:
(1) In addition to the components that are required in an ordinary field emission cold cathode element, i.e., the cathode electrode, gate electrode, anode electrode that receives emitted electrons, and independent power sources connected to these components, the above-described elements further necessitate an electrode for current control and a power source that supplies power to this electrode. PA1 (2) As shown in FIG. 1, when current flows in the direction of depth from emitter 106 toward substrate electrode 109, the depth of n-type silicon 103 in which current is controlled by p-type silicon 105 must be formed with a uniform width of 10 .mu.m or more in order to ensure dielectric strength. PA1 (3) If the n-type silicon is formed by a diffusion method, the width of the layer broadens with increasing depth, and this complicates formation of a layer of uniform width. The layer is therefore formed by methods such as ion implantation, but ion implantation must be carried out a number of times because the depth of spreading varies across the transverse direction of the layer.
For example, the device shown in FIG. 1 requires p-type lead electrode 113 for controlling the voltage impressed to p-type silicon 105.
Furthermore, the employment of an active element in the device shown in FIG. 2 necessitates the provision of many peripheral circuits for controlling the base current or voltage, and also necessitates an electrode and power source in addition to the cathode electrode, gate electrode, anode electrode for receiving electrons, and independent power sources connected to these components that are required in an ordinary field emission cold cathode element. In particular, a potential that is a forward voltage compared with the potential at n-type silicon substrate 201 must be impressed to p-type silicon 214, and as a result, the power source cannot be made common with the other power source.
As a result, in a case in which the current is controlled by means of an active element of the prior art, the element increases in size and the number of circuits provided peripheral to the device increase in number, thereby complicating the composition of the device.
As a result, electrons travel through a silicon layer having a depth of 10 .mu.m or more during ordinary operation, and this gives rise to resistance of the silicon layer portion and an increase in the resistance of the rise current of the current-voltage characteristic of the transistor, thereby impeding high-speed operation of the element overall, increasing power consumption, and moreover, increasing temperature of the element when operating at high currents.
In addition, such processes as thickening the film of the implantation mask are also complex.